The windows used in aerospace applications, especially forward-looking surveillance and tactical systems, are one of the most demanding applications for the single crystal or polycrystalline ceramics. They must strongly withstand severe aero-thermal heating, thermal shock resistance, rain drop and sand erosion, etc. Among the most suitable materials characterized by high strength and reasonable thermal conductivity in combination with high optical transmission over a broad wavelength ranges are ZnS, ZnSe, sapphire, and spinel. A few cost-effective and nearly net-shape fabrication processes were developed in the past 20-30 years to fulfill the requirements for successful infrared windows. The most effective commercial infrared window materials include multispectral zinc sulfide and polycrystalline sapphire. In both of these examples, the single and multi-segment windows were produced primarily by time-consuming metallurgical processes and grinding/polishing to achieve the required precise optical tolerance and good transparency in both the visible and the infrared bands.
As surveillance and tactical applications become more critical at increased flight speeds and over a high range of altitudes and aircrafts, there is a strong need to improve the performance of the infrared systems to provide higher quality signal transmission and advanced imaging overcoming aero-optical limitations.